Switch Mode Power Supply Basics Tutorial

- summary or tutorial about the basics of switch mode power supplies, their design, operation and advantages and disadvantages.

Switch mode power supplies are widely used because of the advantages they offer in terms of size, weight, cost, efficiency and overall performance.

As a result of their performance, switch mode power supplies are used in all but the most exacting applications to provide an efficient and effective source of power for most forms of electronic systems.

Switch mode power supplies have become an accepted part of the electronics scene and are often referred to as switch mode power converters, or just switchers.

Switch mode power supply terminology

Switch mode power supply, SMPS, technology may be referred to by a number of similar terms. While they all look at the same basic technology, they refer to different elements of the overall technology:

  • Switch mode power supply, SMPS:   The term switch mode power supply is generally used to indicate an item that can be connected to the mains, or other external supply and used to generate the source power. In other words it is a complete power supply.
  • Switch mode regulator:   This typically refers just to the electronic circuit that provides the regulation. A switch mode regulator will be part of the overall switch mode power supply.
  • Switch mode regulator controller:   Many switch mode regulator integrated circuits do not contain he series switching element. This will be true if the current or voltage levels are high, because an external series switching element would be able to better handle the higher current and voltage levels, as well as the resultant power dissipation.

Switch mode power supply basics

The basic concept behind a switch mode power supply or SMPS is the fact that the regulation is undertaken by using a switching regulator. This uses a series switching element that turns the current supply to a smoothing capacitor on an off.

Diagram showing the fundamental concept behind a switch mode power supply, SMPS.
Switching regulator concept used in switch mode power supply

The time the series element is turned on is controlled by the voltage on the capacitor. If it is higher than required, the series switching element is turned off, if it is lower than required, it is turned on. In this way the voltage on the smoothing or reservoir capacitor is maintained at the required level.

Switch mode power supply advantages / disadvantages

The use of any technology is often a careful balance of several advantages and disadvantages. This is true for switch mode power supplies which offer some distinct advantages, but also have their drawbacks.

SMPS advantages

  • High efficiency:   The switching action means the series regulator element is either on or off and therefore little energy is dissipated as heat and very high efficiency levels can be achieved.
  • Compact:   As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be made more compact.
  • Flexible technology:   Switch mode power supply technology can be sued to provide high efficiency voltage conversions in voltage step up or "Boost" applications or step down "Buck" applications.

SMPS Disadvantages

  • Noise:   The transient spikes that occur from the switching action on switch mode power supplies are one of the largest problems. The spikes can migrate into all areas of the circuits that the SMPSs power if the spikes are not properly filtered. Additionally the spikes or transients can cause electromagnetic or RF interference which can affect other nearby items of electronic equipment, particularly if they receive radio signals.
  • External components:   While it is possible to design a switch mode regulator using a single integrated circuit, external components are typically required. The most obvious is the reservoir capacitor, but filter components are also needed. In some designs the series switch element may be incorporated within the integrated circuit, but where any current is consumed, the series switch will be an external component. These components all require space, and add to the cost.
  • Expert design required:   It is often possible to put together a switch mode power supply that works. To ensure that it performs to the required specification can be more difficult. Ensuring the ripple and interference levels are maintained can be particularly tricky.
  • Costs:   Careful consideration of the costs of a switch mode power supply must be made before designing or using one. Beyond the basic power supply, additional filtering may be required and this can add to the cost.

Despite the disadvantages, switch mode power supply technology is the major form of power supply technology used for a whole variety of applications especially those included in computers. For applications where very low noise are required, linear regulator technology is still widely used.

By Ian Poole

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